Viscosifying or thickening aqueous and organic solvents to control the viscosity and rheological behavior of a system using them as diluents can be achieved by using high molecular weight polymers. High molecular weight cationic and anionic polymers or polyelectrolytes are known to be useful as thickening agents in water-based systems. However, conventional polymeric thickeners or viscosification agents are generally poor in effectively viscosifying solutions having a high ionic strength or high salt content. This is especially true for polyelectrolytes such as sodium acrylate homopolymers and copolymers that are used as alkali-soluble thickeners.
Polyelectrolytes achieve high solution viscosity and give a viscosification effect largely through static electric repulsion of the same charges on the polymer chains for an extended dimension in the solution. The extension of the polymer chains tends to collapse under high ionic strength conditions.
U.S. Pat. No. 4,497,923 discloses a process for the viscosification of an organic liquid by mixing an organic solution of anionic polymer (sulfonated ethylene-propylene ionomer) and an organic solution of cationic polymer (copolymer of styrene and vinyl pyridine) to form an interpolymer complex in the organic liquid. The organic solution of the interpolymer complex possesses a higher viscosity than the mean of the separate viscosities of the starting solutions at relatively low solids content and exhibits a shear thickening effect desirable for the intended applications such as antimisting of the solvent. This patent is related to non-aqueous systems in the absence of inorganic salts.
U.S. Pat. No. 4,540,496 (to Exxon) discloses a new family of viscosification agents based on terpolymers of acrylamide(AAm)-sodium styrene sulfonate (SSS)-acrylamidopropytrimethyl ammonium chloride (MAPTAC) for high ionic strength drilling muds. The intramolecular ionic complexing gives improved viscosification power for high ionic strength oil drilling fluids. This patent is not related to interpolymer complexes.
U.S. Pat. No. 4,584,339 (to Exxon) discloses a method for increasing the viscosity of water by an interpolymer complex of a low charge ionic acrylamide copolymer (copolymer of acrylamide and sodium styrene sulfonate and a copolymer of acrylamide and methacrylamidopropyltrimethylammonium chloride. To avoid phase separation of the complex in solution, the charge density along the polymer backbones should be relatively low. The mechanism for the observed viscosity enhancement is believed to be due primarily to the increase in the apparent molecular weight of the complex through formation of intermolecular ionic linkages. Although the viscosity of the individual copolymer components shows normal shear thinning behavior, the water solution of the soluble interpolymer complexes exhibits dilatant (shear thickening) rheological behavior. Upon addition of water-soluble inorganic salts the complex can be disturbed and the viscosity reduced.
U.S. Pat. No. 4,942,189 (to Exxon) teaches interfacial viscosification of aqueous solutions utilizing an interpolymer complex of a neutralized sulfonated polymer (water-insoluble) and a copolymer of styrene/vinylpyridine in a water-insoluble organic solvent. When the organic interpolymer complex solution is mixed with water, a large increase in viscosity is achieved due to the formation of a large number of minute water-filled particles dispersed in the continuous aqueous phase. This patent is related to emulsion systems.
U.S. Pat. No. 4,970,260 (to Exxon) discloses a low charge density polymer complex solution for viscosification of salt-containing solutions. The polymer complex consists of a copolymer of acrylamide with a sulfonate monomer (e.g., sodium styrene sulfonate) as the anionic polymer component and a copolymer of acrylamide with a quaternary ammonium monomer (e.g., methacrylamidopropyltrimethylammonium chloride) as the cationic polymer component. Both the cationic polymer and the anionic polymer for the polymer complex are strong polyelectrolytes with a low charge density (<35% quaternary ammonium monomer for the cationic polymer and <50% sulfonate monomer for the anionic polymer). The claimed polymer solutions contain less than 5% of the low charge density interpolymer complex. The patentees stated that high-charge density interpolymer complexes are rather insoluble in salt-containing solutions and therefore have poor viscosification properties and thickening efficiency. The possibility of forming interpolymer complexes by combination of one strong polyelectrolyte with an oppositely charged weak polyelectrolyte and the combination of two oppositely charged weak polyelectrolytes were not explored and disclosed.
EP-A 0 130 732 (to Halliburton) discloses an anionic polymer composition and its use in stimulating a subterranean formation for enhanced oil recovery. The anionic polymer composition contains <20% of a cationic or amphoteric polymer as dispersing agent in order to prevent the agglomeration of the water-soluble polymers. The anionic polymer composition can contain one or more halide salts such as sodium chloride. Synergism of the viscosity increase through combination of an anionic polymer and a cationic polymer was not observed and was not an effect intended or sought by the inventors. Instead, they found that a cationic polymer could be used to disperse the anionic polymer in salt water and solve the problem of the agglomeration of the water-soluble polymer associated with prior art methods.
U.S. Pat. No. 4,839,166 (to L'Oreal) discloses a thickening agent resulting from the ionic interaction of a grafted cationic cellulose and a polymer of methacrylic acid in hair care compositions which do not contain inorganic salts.
U.S. Pat. No. 5,731,034 (to ECC, International) discloses a method of coating paper using a combination of a cationic polyelectrolyte (polyDADMAC) and an anionic polyelectrolyte (polyacrylate) to disperse calcium carbonate pigments (CCP). No thickening effect on the CCP dispersion is reported. In fact, a low viscosity of CCP dispersions at the same solids is desired for the application of the pigment dispersion. CCP is essentially a water-insoluble salt.
U.S. Pat. No. 6,077,887 (to Akzo) discloses a water-soluble polyelectrolyte complex obtained by combining a hydrophobically-modified copolymer of acrylic acid and acrylamide with a hydrophobically-modified cationic cellulose. The desired one-phase (water-soluble) property of the polyelectrolyte complex can only be achieved by use of hydrophobically modified polyelectrolytes. The polyelectrolyte complex is taught to be usable as a thickening agent in water-based systems. No mention is made of its use in salt-containing solutions.
U.S. Pat. No. 4,501,834 (to Colgate-Palmolive) teaches formation of water-soluble and water-insoluble gels by interpolymer reaction of two strong oppositely charged polyelectrolytes, such as poly(2-acrylamido-2-methylpropane sulfonic acid) and DADMAC polymers, in an aqueous medium free of salt. The interpolymer gels exhibiting a significant increase in viscosity are considered to be useful as foam enhancers and in cosmetic compositions such as shaving gels and shampoos. However, this interpolymer complex is not suitable for systems containing salt as the salt interferes with formation of the gel structure.
In many applications, there is a need to viscosity a high salt-containing system, a highly acidic system, or a highly basic system. Such applications include hair color systems in personal care, drilling fluids for enhanced oil recovery, and hard surface cleaning fluids. Conventional polymeric thickeners or viscosification agents are generally poor in effectively viscosifying a high ionic strength or high salt content solution.
One objective of the present invention is to provide a polyelectrolyte complex (PEC) with a synergistic viscosity increase for salt-containing solutions by combination of oppositely charged polymers.
Another objective of the present invention is to provide a high charge density PEC with a synergistic viscosity increase for salt-containing solutions by combination of one strong polyelectrolyte with an oppositely charged weak polyelectrolyte. The cationic polymer for this PEC has a charge density of greater than 35% of cationic monomer for the cationic polymer or >1.6 meq/g active polymer. The anionic polymer for the PEC has a charge density of greater than 50% of anionic monomer for making the anionic polymer or >2.4 meq/g active copolymer.
Still another objective of the present invention is to provide a high charge density PEC with a synergistic viscosity increase for salt-containing solutions by combination of two oppositely charged weak polyelectrolytes.